The interbreeding hypothesis is attractive due to geography if nothing else. Namely, the locations with the last surviving Neanderthal populations (Scandinavia, the English isles) also happen to have the most prevalent rate of Neanderthal appearance genes (fair skin, blond/red hair) of anywhere in the world.

We felt this particular hypothesis to be quite compelling. That Neanderthal glancing at you across the campfire might look a little odd at first, what with her fair skin, auburn hair, and angular cheekbones. But get a couple of Stone Age beers in you and she starts to look like a perfect 10.

Stone Age beer likely played a key role in human and Neanderthal hook ups.
[Image Source: Asle Rønning]

The new study comes in response to a 2012 study in the Proceedings of the National Academy of Sciences by Anders Eriksson and Andrea Manica of the University of Cambridge. That study acknowledged the interbreeding possibility, but argued in favor of an alternative -- sustained substructure. According to that theory, certain subpopulation of each group retained traits, or evolved traits in parallel, via random mutations.

But Professor Laurent Frantz of Wageningen University calls that idea "unparsimonious" (too theoretically complex/violating Occam's razor). He and his UK colleague did a more thorough analysis of known human mutation rates and genetic variance, plus looked at population distribution and other factors. The results, he states, show a resounding win for the interbreeding hypothesis, while leaving the door open to the possibility that some common genes did come from sustained substructure.

Professor Frantz was the study's senior author, UK Professor Konrad Lohse was the first author. Professor Frantz comments to The Verge:

We did a bunch of math to compute the likelihood of two different scenarios. We were able to do that by dividing the genome in small blocks of equal lengths from which we inferred genealogy.

Our analysis shows that a model that involves interbreeding is much more likely than a model where there was sustained substructure in Africa. [Substained substructure might have contributed to genetic commonality] but it cannot be used to explain the genetic similarities [alone].

There seemed to be something that has gone wrong [in that study] because it seems unparsimonious... When we tested two hypotheses, we got a high support for a scenario where humans and Neanderthals interbred.

There have been a lot of arguments about what happened to these species. Some think that we outcompeted [other hominins] or that they were killed by humans, but now we can see that it's not that simple. Human evolution is much more complex than we previously thought.

In other words, why would early man, struggling to survive against nature and competing tribes, waste resources on killing and driving out Neanderthals? They could instead ally with them, recruiting the close relative to join the tribe.

Geneticist Svante Pääbo, one of the men who helped sequence the Neanderthal genome peers at the fossilized remains of his possible ancestor. [Image Source: Frank Vinken]

It's a compelling notion indeed. So crack open a cold beer; chances are your ancestors were doing the same on one fateful night some 500,000 years ago. Thanks to them, now that Neanderthal isn't just your close phylogenetic kin, it's also your direct ancestor.